Light emitting diodes (LEDs) are currently produced using epitaxial crystal growth techniques, such as liquid phase epitaxy (LPE), vapor phase epitaxy (VPE), molecular beam epitaxy (MBE), and metallo-organic chemical vapor deposition (MOCVD). With any of these techniques, epitaxial layers are grown on a substrate. The substrates are generally chosen to be lattice matched with the epitaxial layer. As an example, layers of the material Al.sub.x Ga.sub.1-x As are grown on gallium arsenide (GaAs) substrates because this combination of materials offers good lattice match for all values of x, and good light transmittance.
Unfortunately, a significant drawback exists. In many instances, the substrate may appreciably absorb that wavelength range of photon that the light emitting diode emits. This is true with AlGaAs, GaAsP, and InGaP active regions. This is also true for quantum well heterostructure (QWH) such as in the AlGaAs/GaAs or AlInGaP/InGaP material systems grown on GaAs substrates. The active region composition and thickness determines the wavelength of transmitted light for the light emitting structure. For example, photons produced in the active epitaxially grown AlGaAs region which are directed toward the GaAs substrate are absorbed by the GaAs substrate. Naturally, absorbed photons do not contribute to the device light output. If these otherwise absorbed photons could be utilized, the light emitting efficiency of a light emitting diode would increase substantially.
As a solution to this problem, previous work has been attempted to grow a thick transmissive epitaxial layer above an absorbing substrate, and then subsequently removing the substrate material. By etching the original substrate material away before packaging the light emitting diode, light may be extracted from both the top and bottom surfaces of the device. However, such an approach is impractical since it is time consuming both with respect to the duration of crystal growth and post growth device processing.
An alternative and more practical solution would be to form a reflecting surface above an appropriate substrate prior to selective growth of the light emitting layer. This reflecting surface could be used to increase the light output of the light emitting diode, such as formed by AlGaAs on GaAs or InGaP on GaAs. Such a reflector would redirect those photons directed toward the substrate to the surface of the device. Without redirection, these photons would be lost to the absorbing substrate. Such a reflector could also be used as the rear mirror of a surface emitting laser.
It is foreseeable that selective crystal growth techniques could be used to form such a rear reflecting surface. Selective crystal growth techniques have been used to control the location of epitaxial crystal deposition on patterned materials. The selective crystal growth results in epitaxial growth of the desired material on a patterned substrate without simultaneous material deposition on the pattern forming mask.
Therefore it would be desirable to provide a method for forming a light emitting diode having a rear reflecting surface so as to enhance light reflectance of the light emitting diode. It would also be desirable to provide such a method which utilizes selective epitaxial crystal growth techniques.